Back light module with birefringent crystal assemblies

ABSTRACT

A back light module ( 100 ) includes a light guide plate ( 130 ), a number of light sources ( 110 ) at one side of the light guide plate, and a number of birefringent crystal assemblies ( 120 ) respectively located between the light sources and the side of the light guide plate. Each birefringent crystal assembly includes a generally wedge-shaped first birefringent crystal ( 121 ) and a generally wedge-shaped second birefringent crystal ( 122 ) adhered to each other back-to-back. The birefringent crystal assembly enlarges a radiation angle of optical beams emitted from the corresponding light source. The optical beams with enlarged radiation angles enter the side of the light guide plate, and help provide excellent uniformity of brightness to an associated liquid crystal display panel.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention concerns light source systems, and more specifically aback light module typically used in a liquid crystal display (LCD).

2. Description of the Prior Art

Users of portable electronic devices such as laptop and notebookcomputers, mobile phones and game devices expect the viewing screensthereof to be large, clear and bright. Performance equal to that ofcathode-ray tube (CRT) desktop monitors is desired. LCDs are one type offlat panel display (FPD) which can satisfy these expectations. However,because liquid crystals in an LCD are not self-luminescent, the LCDneeds a back light module which offers sufficient luminance (brightness)for a planar surface.

Referring to FIG. 10, a conventional back light module 20 comprises alight guide plate 22, a plurality of light emitting diodes (LEDs) 21adjacent one side of the light guide plate 22. Light emitted from theLEDs 21 enters the side of the light guide plate 22 and eventually emitsuniformly from a top surface of the light guide plate 22.

Referring to FIG. 11 and FIG. 12, an optical intensity of the LED 21rapidly decreases according to an increase in the angle of emission. Infact, almost all of the optical intensity is concentrated in a smallrange of angles in the vicinity of the 0 degree angle of emission (i.e.,direct emission). In another words, the LED 21 has a very smalleffective radiation angle. When the LED 21 irradiates the side of thelight guide plate 22, some areas of the light guide plate 22 nearmid-points between adjacent LEDs 21 receive almost no light. Thusdarkened areas are formed on the top surface of the light guide plate 22near the side of the light guide plate 22. The back light module 20 doesnot provide uniform brightness over an entire area of the associatedliquid crystal display panel.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a backlight module which affords excellent uniform brightness for anassociated liquid crystal display panel.

To achieve the above object, a back light module of a first preferredembodiment of the present invention includes a light guide plate, aplurality of aligned light sources at one side of the light guide plate,and a plurality of birefringent crystal assemblies respectively locatedbetween the light sources and the side of the light guide plate. Eachbirefringent crystal assembly includes a generally wedge-shaped firstbirefringent crystal and a generally wedge-shaped second birefringentcrystal adhered to each other back-to-back. The birefringent crystalassembly enlarges a radiation angle of optical beams emitted from thecorresponding light source. The optical beams with enlarged radiationangles enter the side of the light guide plate, and help provideexcellent uniformity of brightness to an associated liquid crystaldisplay panel.

These and other features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription and claims, and from the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of a back light module in accordance withthe first preferred embodiment of the present invention, the back lightmodule comprising a plurality of birefringent crystal assemblies;

FIG. 2 is an enlarged, isometric view of a first birefringent crystal ofany one of the birefringent crystal assemblies of FIG. 1, but viewedfrom another aspect;

FIG. 3 is an enlarged, isometric view showing optical axes of the twowedge birefringent crystals of the birefringent crystal assembly of FIG.1;

FIG. 4 is an enlarged, side elevation of the first birefringent crystalof FIG. 2, showing an essential optical path thereof;

FIG. 5 is an enlarged, side elevation of a second birefringent crystalof any one of the birefringent crystal assemblies of FIG. 1, showing anessential optical path at a second surface thereof;

FIG. 6 is similar to FIG. 5, but showing an essential optical path at afirst surface of the second birefringent crystal;

FIG. 7 is a graph showing an angle deviation of optical beams emittedfrom any one of the birefringent crystal assemblies of FIG. 1 varyingaccording to a wedge incline angle of the first and second birefringentcrystals of the birefringent crystal assembly;

FIG. 8 is a graph of luminance varying according to radiation angle,thus showing an intensity distribution of the optical beams emitted fromany one of the birefringent crystal assemblies of FIG. 1;

FIG. 9 is an isometric view of a back light module in accordance with asecond preferred embodiment of the present invention;

FIG. 10 is an isometric view of a conventional back light module; and

FIG. 11 is a graph showing an intensity distribution of optical beamsemitted from an LED of the back light module of FIG. 10 varyingaccording to an angle of radiation from the LED.

FIG. 12 is a graph of luminance varying according to radiation angle,thus showing an intensity distribution of the optical beams emitted fromthe LED of the back light module of FIG. 10.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

For facilitating understanding, like components are designated by likereference numerals throughout the various embodiments of the inventionas shown in the various drawing figures.

Reference will now be made to the drawing figures to describe thepresent invention in detail.

Referring to FIG. 1, a back light module 100 of the first preferredembodiment of the present invention comprises a light guide plate 130made of a transparent material, a plurality of LEDs 110 aligned parallelto an optical input surface 131 of the light guide plate 130, and acorresponding plurality of birefringent crystal assemblies 120respectively located between the LEDs 110 and the optical input surface131. Radiation angles of optical beams emitted from the LEDs 110 areenlarged by the birefringent crystal assemblies 120, and the opticalbeams then enter the light guide plate 130 through the optical inputsurface 131.

The light guide plate 130 is a substantially rectangular plane body, andcomprises the optical input surface 131, the optical output surface 132,a bottom surface 133, and three side surfaces (not labeled). A pluralityof reflective dots 134 are formed on or applied to the bottom surface133, to promote uniform emission of light from the optical outputsurface 132. Further, to improve optical efficiency, reflective sheetsor films can be attached on the bottom surface 133 and the sidesurfaces.

Referring also to FIG. 2 and FIG. 3, each birefringent crystal assembly120 is substantially a parallelepiped. The birefringent crystal assembly120 includes a generally wedge-shaped first birefringent crystal 121 anda generally wedge-shaped second birefringent crystal 122 adhered to eachother. The birefringent crystals 121, 122 have substantially identicalstructures, and respectively comprise a first surface 1211, 1222 and asecond surface 1212, 1221. The first surface 1211, 1222 is oblique toand opposite from the second surface 1212, 1221, which is substantiallyparallel to the optical input surface 131 of the light guide plate 130.The two second surfaces 1212, 1221 are adhered to each other therebyforming the birefringent crystal assembly 120. Each first birefringentcrystal 121 has an optical axis A on a plane MPONM thereof, the opticalaxis A forming a 45 degree angle with respect to an MP direction. Eachsecond birefringent crystal 122 has an optical axis B perpendicular tothe optical axis A of the corresponding first birefringent crystal 121.

Referring to FIGS. 4-6, in operation, an angle between the first surface1211 and the second surface 1222 is defined as θ_(w). When an opticalbeam emitted from each LED 110 enters the corresponding first surface1211, the birefringence characteristics of the first birefringentcrystal 121 separate the optical beam into two beams: an o-ray and ane-ray. The o-ray and the e-ray have polarization planes perpendicular toeach other, and therefore the two polarized light beams have separateoptical paths. As regards the optical path of the o-ray, when theoptical beam enters the first surface 1211 of the first birefringentcrystal 121 (shown in FIG. 4), the incident angle is θ_(w), therefractive index for the o-ray is n_(o), the refractive index of air isdesignated as 1, and the refractive angle is θ_(o1). The relationshipbetween the incident angle and the refractive angle is:sin θ_(w)=n_(o)sin θ_(o1)When the o-ray is incident on the interface of the second surface 1212of the first birefringent crystal 121 and the second surface 1221 of thesecond birefringent crystal 122 (shown in FIG. 5), the incident angleθ_(o2) is equal to θ_(w) minus θ_(o1). The refractive index of theincident medium is n_(o) because the second birefringent crystal 122 hasan optical axis B perpendicular to the optical axis A of the firstbirefringent crystal 121, and therefore the o-ray in the firstbirefringent crystal 121 has the characteristic of an e-ray in thesecond birefringent crystal 122. The refractive index of the emittingmedium is n_(e), and he refractive angle is θ_(o3). The relationshipbetween the incident angle and the refractive angle is:n _(o) sin(θ_(w)−θ_(o1))=n _(e) sin θ_(o3)When the o-ray is incident on the first surface 1222 of the secondbirefringent crystal 122 (shown in FIG. 6), the incident angle θ_(o4) isequal to θ_(w) minus θ_(o3), the refractive index of the incident mediumis n_(e), the refractive index of the emitting medium (air) isdesignated as 1, and the refractive angle is θ_(o5). The relationshipbetween the incident angle and the refractive angle is:n _(e) sin(θ_(w)−θ_(o3))=sin θ_(o5)Then when the o-ray emits from the birefringent crystal assembly 120, itforms an angle θ_(o6) relative to a plane that is parallel to a bottomsurface of the birefringent crystal assembly 120, wherein θ_(o6) isequal to θ_(w) minus θ_(o5).

As regards the optical path of the o-ray produce by the firstbirefringent crystal 121, after passing through the birefringent crystalassembly 120, the resulting e-ray also forms an angle relative to theplane that is parallel to the bottom surface of the birefringent crystalassembly 120. Referring to FIGS. 7 and 8, the result is that thebirefringent crystal assemblies 120 provide excellent enlargement of theradiation angles of the optical beams emitted from the LEDs 21. Theoptical beams with enlarged radiation angles enter the light guide plate130 through the optical input surface 131, and help provide excellentuniformity of brightness to an associated liquid crystal display panel(not shown). The optical intensity curves 141, 142 represent the splitbeams, and optical intensity curve 143 represents the joined beam so asto increase the radiation angle.

Referring to FIG. 9, a back light module 300 of the second preferredembodiment of the present invention has a structure similar to that ofthe back light module 200. The difference is that the back light module300 comprises two long birefringent crystal assemblies 320 positionedadjacent two opposite ends of the light guide plate 130 respectively.Each birefringent crystal assembly 320 couples with a plurality ofcorresponding aligned LEDs 110, and comprises two generally wedge-shapedbirefringent crystals adhered to each other.

In the present invention, any of the birefringent crystals can belithium niobate (LiNbO₃) or yttrium orthovanadate (YVO₄) crystals. Thegreater the birefringent effect that the birefringent crystal has, themore uniform the brightness of the back light module can be. Further,another kind of light source having similar characteristics to those ofLEDs can be used instead of the LEDs.

It is to be understood, however, that even though numerouscharacteristics and advantages of the present invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, the disclosure is illustrativeonly, and changes may be made in detail, especially in matters of shape,size and arrangement of parts within the principles of the invention tothe full extent indicated by the broad general meaning of the terms inwhich the appended claims are expressed.

1. A back light module comprising: a light guide plate; a plurality oflight sources at one side of the light guide plate; and at least onebirefringent crystal assembly located between the light sources and thelight guide plate.
 2. The back light module as described in claim 1,wherein the back light module comprises a plurality of the birefringentcrystal assemblies, and a number of the birefringent crystal assembliesis equal to a number of the light sources.
 3. The back light module asdescribed in claim 1, wherein said birefringent crystal assemblycomprises two generally wedge-shaped birefringent crystals attached toeach other.
 4. The back light module as described in claim 3, whereinthe birefringent crystals are lithium niobate crystals.
 5. The backlight module as described in claim 3, wherein the birefringent crystalsare yttrium orthovanadate crystals.
 6. The back light module asdescribed in claim 3, wherein an angle between optical axes of the twobirefringent crystals is substantially 90 degrees.
 7. The back lightmodule as described in claim 1, wherein said birefringent crystalassembly is substantially a parallelepiped.
 8. The back light module asdescribed in claim 1, wherein the light sources are light emittingdiodes.
 9. A light module comprising: a light guide plate; at least onepoint light source located beside said light guide plate; at least onelight diverging/converging device located between said point lightsource so as to initially split light and successively rejoin splitlight before the light enters the light guide plate.
 10. The lightmodule as described in claim 9, wherein said split light includes anO-ray and an E-ray.
 11. The light module as described in claim 9,wherein each of split light performs an unsymmetrical optical intensitycurve with regard to a symmetrical radiation angle range while therejoin light performs a symmetrical optical intensity curve with regardto the symmetrical radiation angle range.
 12. The light module asdescribed in claim 11, wherein the symmetrical optical intensity curveis configured to be shorter and fatter than an original intensity curvederived from the point light source which is not affected by said lightdiverging/converging device.
 13. A light module comprising: a lightguide plate; at least one point light source located beside said lightguide plate; at least one light transforming device located between saidpoint light source so as to convert the one point light source into atleast two virtual point light sources, thus increasing a light radiationangle range.
 14. The light module as described in claim 13, wherein amaximum optical intensity of a transformed light is smaller than that oflight which is not effected by said light transforming device.